During the past two decades, there has been a birth of interest in a class of devices commonly referred to as electroluminescent displays which are solid state devices that use the direct production of light in thin films to provide displays of satisfactory appearance at moderate cost.
This technology led to the development of liquid crystal displays based upon a phenomenon known as dynamic scattering wherein current is passed through a nematic liquid crystal, so causing the material to break up into domains having randomly directed axes. The next stage in the development of such devices involved the fabrication of a twisted nematic display in which a nematic liquid crystal is disposed intermediate two closely spaced glass plates coated with a polymer and rubbed such that the nematic liquid crystal is aligned parallel to the rubbing direction. If the plates are robbed at 90 degree angles, the liquid crystal deforms into a twisted structure. Passage of polarized light through the display results in the plane of polarization following the twist. The application of an electric field to the display permits the liquid crystals to rotate and align themselves with the field, thereby leading to disruption of the twist and rotation of polarization. This permits either full transmission or no transmission in the display.
Later developments in the field led to the generation of thin film transistor/liquid crystal displays and to color liquid crystal displays. In the case of the latter, workers in the art have recognized that a reduction in efficiency is ofttimes encountered by absorption of light in the color filters. Accordingly, efforts have continued to alleviate or eliminate this limitation.
Heretofore, the most common technique for overcoming this problem has been to design an active matrix liquid crystal display including a glass substrate (front glass plate) having a black matrix typically comprising chromium oxide deposited upon a portion thereof. The purpose of this matrix is to define and outline individual pixels which is of particular importance for color displays in providing a separator between the elements of the color filter from each other.
Additionally, the black matrix serves to mask the electronics on the back plate of the device, thereby providing protection therefor from ambient light and associated photodegradation.
Black matrix structures employed heretofore typically comprise a grid of black lines, usually of the order of 20 nanometers in thickness spaced so as to provide an array of open opaque rectangles, typically 100 .mu.m on a side. The black matrix is also chosen to minimize reflection of light which tends to result in the degradation of the display.
In the past, numerous approaches for attaining this end have been employed by those skilled in the art. Thus, for example, black matrixes have included grids made from (a) dyed black photoresists, (b) photoresists impregnated with carbon black, (c) patterned chromium, (d) patterned chromium/chromium oxide, and (e) patterned chromium/chromium oxide/chromium trilayers. Both the bilayer and the trilayer structures have been engineered to minimize reflectance of the matrix. Studies have revealed that the best of the matrixes among these prior art structures evidences an average reflectance of approximately 3%. Accordingly, efforts have continued in the search to enhance this characteristic.